1. Field of the Invention
This invention relates to industrial process control systems of the general type disclosed in U.S. Pat. application Ser. No. 833,977, filed on Sept. 16, 1977, by M. P. Freitas et al. More particularly, this invention relates to an improved swept-carrier data transmission system providing privacy and resistance to various interferences such as noise or willful or accidental jamming, and especially to a frequency-tracking receiver for use in such a system.
2. Description of the Prior Art
Frequency-tracking receivers commonly have an internal oscillator with a center frequency differing from the received carrier but of similar sweep. As the received carrier and the output of the internal oscillator are mixed (heterodyned), an intermediate frequency (IF) carrier is obtained with the sweep reduced to a sweep error signal but with the data modulation remaining intact. The result is that the IF carrier may be filtered through a bandwidth essentially dictated by the data modulation and the sweep error signal.
Alternatively, the received carrier can be passed through a narrow-band filter the center frequency of which tracks the center frequency of the received carrier. Here again, the received carrier of a wide frequency sweep is filtered through a narrower IF bandwidth.
Generally, frequency tracking receivers fall into two categories:
(1) In the first category are receivers arranged to reduce all frequency deviations from the center frequency of the carrier. Thus, for example, a carrier of a complex non-predescribed deviation will have its total deviation tracked at each instant. Applications of this type include frequency modulation (FM) demodulation at a lowered noise threshold, and a reduction of incidental FM in amplitude modulation (AM) transmissions. (2) In the second category are receivers arranged to track only the moving center frequency of a carrier; any other components of the frequency deviation, if present, are not to be tracked. Applications of this type are found in communication systems utilizing a varying carrier center frequency for privacy or resistance to jamming and noise.
Receivers for these two categories of tracking differ in important ways. One example of the first category, often referred to in the literature as the "FM Feedback Loop", is disclosed by Guanella in U.S. Pat. No. 2,206,695, and its design is discussed extensively in J. Klapper and J. Frankle, Phase-Locked and Frequency-Feedback Systems, New York: Academic Press, 1972. In systems of the second category, frequency and phase synchronization has been effected by means of synchronization pulses transmitted through a separate channel as taught by Kendall in U.S. Pat. No. 1,592,940, or by means of synchronization pulses produced in the receiver by the movement of the swept carrier through a fixed narrow-band IF filter as taught by Silver, et al, in U.S. Pat. No. 2,448,055.
U.S. application Ser. No. 790,156, filed Apr. 22, 1977, by Victor A. Bennett, Jr. for "Continuously-Synchronized Tracking Receiver for A Priori Defined Swept Carriers", and assigned to the assignee of the present invention, discloses a receiver utilizing a filtered carrier signal to drive a frequency discriminator circuit which produces a sweep error signal. A phase-locked loop responsive to this error signal develops a sweep frequency signal which is used to control the receiver sweep in a continuous manner.
The known prior art referred to above suffers from important disadvantages which the present invention overcomes or significantly minimizes.